As a positive electrode plate for an alkaline storage battery, there has conventionally been proposed a positive electrode plate comprising nickel hydroxide particles carried on a foamed nickel substrate having three-dimensionally interconnected pores and a porosity of about 95% as shown in FIG. 1 (see, for example, Japanese Laid-Open Patent Publication No. Sho 50-36935 (Patent Document 1)). Currently, a foamed nickel substrate is widely used as a core material for positive electrode plate in high-capacity alkaline storage batteries. When a battery containing such positive electrode plate is repeatedly charged and discharged, however, the positive electrode plate expands in the thickness and width directions. This is caused by expansion of the volume of the nickel hydroxide contained in the positive electrode plate. When a positive electrode active material extended in the width direction separates therefrom, there arises a problem that the separated active material comes in contact with a negative electrode plate positioned opposite to the positive electrode plate. Likewise, in the negative electrode plate, problems may occur such as separation of the active material and the separated active material coming in contact with the positive electrode plate positioned opposite to the negative electrode plate.
There is recently proposed a core material having a two-dimensional structure such as a punched metal sheet or expanded metal sheet. Core materials having a two-dimensional structure are usually inexpensive because they are produced by a mechanical punching method. The use of the two-dimensionally structured core material can produce a positive electrode plate with a higher capacity. Alternatively, the use of a core material comprising an electrolytic foil can produce a thinner electrode plate. The two-dimensionally structured core materials, however, have the drawback that they have poor function to hold an active material. In view of the above, there is proposed to process the two-dimensionally structured core material into a three-dimensionally structured one (see, for example, Japanese Laid-Open Patent Publications No. Hei 7-130370 (Patent Document 2) and No. 2002-15741 (Patent Document 3)).
FIG. 2 shows a perspective view of a three-dimensionally processed core material. FIG. 3 shows an enlarged view of a part of FIG. 2. This core material 1 comprises a metal sheet 3 having slits formed in a matrix arrangement. Each strip between a pair of the slits forms a first curved protrusion 4 or a second curved protrusion 7. The first curved protrusions 4 and the second curved protrusions 7 protrude alternately above and below the metal sheet 3 along a direction indicated by X. The first curved protrusions 4 and the second curved protrusions 7 are arranged parallel to each other to form a protrusion row 8 along the direction X. A plurality of protrusion rows 8 are arranged along the other direction Y, which is perpendicular to the direction X, with a flat part 9 having a given width between each protrusion row 8. The direction Y is in the longitudinal direction of the core material 1, and the direction X is in the width direction of the core material 1. A plain part 5 where the curved protrusions 4 and 7 are not formed is left on each of the edges that are along the longitudinal direction of the core material 1. On the plain part 5 are formed a plurality of groove-shaped recesses 15 in an undulatory pattern such that the plurality of groove-shaped recesses 15 are parallel to each other.
Even when an active material is carried on the core material as shown in FIGS. 2 and 3, however, the retention of the active material is not satisfactory. As such, if the volume of the nickel hydroxide contained in the positive electrode plate expands due to repetition of charge and discharge, the positive electrode plate will expand in the thickness and width directions. Accordingly, a problem similar to the one encountered in the case of using a foamed nickel substrate arises.
Meanwhile, in order to prevent short-circuiting caused when the burrs of a core material pierce a separator, there is proposed to cover the edges of an electrode plate with a resin. Each edge includes an end face and peripheral sides of the end face. (see, for example, Japanese Laid-Open Patent Publication No. Hei 5-190200 (Patent Document 4)). This proposal is not intended to prevent an active material from separating. Accordingly, it has almost no effect of preventing an active material from separating from the end faces covered with a resin.